1. Field of the Invention
The present invention relates to a bitmap structure for reporting reception result of packets applying a retransmission technique and a method for transmitting/receiving the reception result in a mobile communication system.
In general, a radio channel can cause errors in transmitted packets under the influence of multi-path fading, interferences among users, noises, and so forth. A solution to this problem includes a Forward Error Correction Code (FEC) scheme in which the probability of error occurrence is lowered by additionally sending redundant information, an Automatic Repeat Request (ARQ) scheme in which, when errors occurs, retransmission of packets where the errors have occurred is requested, and a Hybrid Automatic Retransmission Request (HARQ) scheme which combines both the schemes.
In the ARQ scheme, a receiver uses an Acknowledgment (ACK)/Not Acknowledgment (NACK) signal for notifying a transmitter of whether or not received packets are erroneous. The ACK signal confirms to the transmitter that the receiver has received the corresponding packets. In contrast , the NACK signal confirms to the transmitter that the receiver has failed to receive the corresponding packets. If the transmitter receives the NACK signal, the transmitter retransmits the corresponding packets to the receiver.
In addition to the general ARQ scheme in which reception results are acknowledged on a packet-by-packet basis, there is a block ARQ scheme in which reception results of a plurality of transmitted packets are acknowledged as a group through a block ARQ message.
FIG. 1 is a diagram illustrating a basic concept of a general block ARQ scheme based on an example which presumes that the block ARQ scheme is applied to every three packets.
Referring to FIG. 1, a transmitter transmits three packets, that is, Packet #1, Packet #2 and Packet #3, in sequence. The three packets (Packet #1 to Packet #3) have the same Destination Address (DA), for example, DA2. Each of the packets (Packet #1 to Packet #3) is provided with a Sequence Number (SN) and a Fragmentation Number (FN). The SN signifies the order in which packets are transmitted from an upper layer. Even packets having the same SN may be transmitted over a plurality of packets as occasion demands. The FN signifies the order of transmitting the plurality of packets divided over the transmission from packet having the one same SN.
A receiver checks whether or not packets are continuously received and which packets are not received by comparing the SN and the FN of a received packet with those of previously received packets. In the following description, packets at an SN level will be referred to as ‘SN level packets’, and packets divided from the SN level packets will be referred to as ‘fragmentation packets’. When a packet is not referred to as the SN level packet or the fragmentation packet, but simply referred to as ‘a packet’, it is meant to incorporate both of the above-mentioned two types of packets.
Of the three packets, the first and second packets (Packet #1, Packet #2) are fragmentation packets having the same SN (e.g., SN 1) and different FNs (e.g., Frag 1, Frag 2). The third packet (packet #3) is an SN level packet having a different SN (e.g., SN 2) from that of the first and second packets (Packet #1, Packet #2).
In FIG. 1, it is assumed that the receiver succeeds in receiving the first and third packets (Packet #1, Packet #3) and fails to receive the second packet (Packet #2).
The receiver configures a block ACK message on the basis of the reception result as stated above and transmits the configured block ACK message to the transmitter. The block ACK message includes a header and a payload. A Destination Address DA1 is recorded in the header. The Destination Address DA1 is an address of the transmitter. The reception results for the respective received packets are recorded in the payload.
Applying the above-mentioned assumption, the ACK information is recorded as the reception result corresponding to the first and third packets (Packet #1, Packet #3), and NACK information is recorded as the reception result corresponding to the second packet (Packet #2). SNs and FNs of the corresponding packets are recorded together in the reception results.
The transmitter receives the block ACK message. The transmitter confirms through the block ACK message that the receiver succeeds in receiving the first and third packets (Packet #1, Packet #3) and failed to receive the second packet (Packet #2). Thereafter, although not shown in FIG. 1, the transmitter retransmits the second packet (Packet #2).
The above-mentioned scheme in which the reception results for all the received packets are recorded in one block ACK message can be realized in various ways. However, in order to use a message having the shortest length, a bitmap scheme is employed.
FIGS. 2 to 4 show examples of using the bitmap scheme for acknowledging reception results.
Referring to FIG. 2, the block ACK message includes a block ACK starting sequence field and a bitmap field. The bitmap field consists of N ACK report fields. ‘N’ is a value corresponding the maximum SN and signifies the maximum number of sequences capable of being acknowledged. That is, ‘N’ may be defined as the maximum allowable number of SN level packets which can be processed by one block ACK message.
The first SN level packet, with which a bitmap in a corresponding message deals, is recorded in the block ACK starting sequence field. Each of reception results for N consecutive packets starting from the packet having the SN recorded in the block ACK starting sequence field is recorded in the bitmap Field.
The respective ACK report fields constituting the bitmap field are divided into (M×8) regions b0, b1, b2, . . . , b(n), . . . , b(8×M−1) corresponding to the number of fragmentation packets which can be divided to the maximum extent from one SN level packet. Hereinafter, such regions b0, b1, b2, . . . , b(n), . . . , b(8×M−1) will be referred to as ‘reception result information fields’. This is because reception results are acknowledged on a packet-by-packet basis. Thus, if the reception result is expressed by one bit, M octets are required for the total reception results information fields for one SN level packet, and so the bitmap field has an overall length of M×N octets.
For example, when SN=1 is recorded in the block ACK starting sequence field, the reception result for a fragmentation packet having SN=1 and FN=n−1 will be recorded in the reception result information field b(n) 210. If the receiver succeeds in receiving this fragmentation packet, ‘1’ is recorded in the reception result information field b(n) 210. Otherwise, if the receiver fails in receiving the fragmentation packet, ‘0’ is recorded in the reception result information field b(n) 210. This is based on the assumption that ‘1’ is an indicator bit representing reception success and ‘0’ is an indicator bit representing reception failure. As another example, when ‘5’ is recorded in the block ACK starting sequence field, ‘1’ is set to a third bit of the second octet if a fragmentation packet having SN=6 and FN=3.
FIG. 3 shows the above-mentioned general example when applied to a system based on the IEEE 802.16 standard (the 802.16), and FIG. 2 shows the same example when applied to a system based on the IEEE 802.11e standard (the 802.11e).
A block ACK message shown in FIG. 3 includes a connection ID field, an ACK control field and a plurality of ACK MAP fields. The ACK control field includes a field in which a starting SN is recorded, and a field in which the number of ACK MAPs (m) is recorded. The ACK MAP fields are equal in number to the number of ACK MAPs (m). The ACK MAP field has the same structure as that of the ACK report field in FIG. 2. In FIG. 3, each of the connection ID field, the ACK control field and the plurality of ACK MAP fields are configured as a 2-octet field. Thus, the block ACK message has an overall length of ‘(m+2)×2’. Usually, ‘m’ is a variable value and the maximum number of fragmentation packets is 16 in the 802.16.
A block ACK message shown in FIG. 4 includes a BA starting sequence control field and a BA bitmap field. Information indicating a starting sequence recorded in the bitmap field is recorded in the BA starting sequence control field. The BA bitmap field consists of a plurality of ACK MAP fields. Each ACK MAP field has the same structure as that of the ACK report field in FIG. 2. For example, in the 802.11e, it is possible to simultaneously perform ACK processing for a maximum of 64 SN level packets, and one SN level packet can be divided into 16 fragmentation packets. Thus, when each ACK MAP field is configured as a 2-octet field, the BA bitmap field must maintain a size of 128 octets.